The present invention relates generally to devices and methods for casting of engine components, and more particularly to an advanced bi-metallic piston and a way to manufacture the same.
Pistons used in internal combustion engines are typically made up of a head (also called a dome), skirts, one or more ring grooves, and lands between the grooves. While they are subjected to high combustion temperatures and pressures during engine operation, increasingly stringent emissions and efficiency requirements dictate that pistons of the future will need to be designed to withstand even more demanding operating conditions. This in turn will necessitate the use of higher-temperature capable materials and damage-resistant designs. This is especially true for pistons used in diesel engines that, in addition to being the predominant engine form for larger, commercial vehicles, are increasingly being used to power passenger vehicles.
While all of the various piston components mentioned above are expected to be subjected to additional loads as more power is extracted from smaller structures, it is the dome which, by virtue of being directly exposed to the combustion process, can be expected to be particularly vulnerable to thermo-mechanical damage. Unfortunately, light alloys typically used for pistons tend to have limited mechanical and temperature capability. For example, while aluminum alloys have conventionally been used for weight reduction in diesel engine pistons, their limited thermal and mechanical durability makes them incompatible with the higher temperature requirements of a more complete (and therefore higher temperature) combustion process. Steel pistons have the capability to endure the extreme environment; however, they are heavier than aluminum pistons. This weight problem is exacerbated by the high rate of speed and acceleration associated with piston movement, meaning that ancillary structures may additionally have to be fortified, with an even more detrimental weight impact.
Attempts have been made to combine the heat resistance of high temperature-capable materials with the lower weight of aluminum-based materials in diesel pistons. However, although composite pistons may satisfy the above objectives, the difficulties associated with their manufacture have offset many of their benefits. This is especially so because pistons have long been made as cast parts with some post-casting machining or related modifications. As such, it has been difficult to combine the inherent low-cost approach of casting with the flexibility of tailored material placement in the piston.
Currently, the piston is cast with the riser at one side of the dome, which results in property variation around the dome. The casting method may be changed to feed the piston so that the riser is in a more typical location, such as at the center of the dome. In another alternative, the bowl rim can be re-melted using TIG or lasers. While this may achieve a particularly well-refined microstructure in the dome region of interest, there is no insulation benefit from the heat of combustion.